1. Field of the Invention
The present invention relates to an ion source to be used to an ion implantation apparatus for producing, for example, a semi-conductor device, using an organometallic gas as a raw gas.
2. Description of the Related Art
This kind of a conventional ion source is shown in FIG. 3. The similar ion source as this is described in JP-A-9-35648.
This ion source is called as an electron impact ion source, and more specifically a Bernus type ion source. The ion source is furnished with a plasma production container 2 also serving as an anode, a filament 8 (hot cathode) equipped at one side within the plasma production container 2, a reflecting electrode 10 equipped at the other side within the same, and an ion leading slit 4 provided in the wall of the plasma production container 2. In the vicinity of an outlet of the ion leading slit 4, a leading electrode 14 is provided for leading ion beam 16 from the plasma 12 produced within the plasma production container 2. Outside of the plasma production container 2, a magnetic field generator 18 is disposed for generating magnetic field B in the axial direction thereof. Numerals 24 and 25 designate insulating materials.
Into the plasma production container 2, an organometallic gas 28 is introduced as a raw gas (source gas) for making a plasma 12 and ion beam 16. The organometallic gas 28 is introduced through a gas-introducing inlet 6 provided in the wall of the plasma production container 2 and a gas introducing pipe 26 connected thereto.
The organometallic gas 28 is, for example, gaseous trimethylindium [In(CH3)3], triethylindium [In(C2H5)3], trimethylgallium [Ga(CH3)3], triethylgallium [Ga(CaH5)3] or trimethylantimony [Sb(CH3)3].
In such an ion source, the inside and the outside of the plasma production container 2 is air-exhausted by vacuum. The filament 8 is heated by a filament electric source 20. The organometallic gas 28 is introduced into the plasma production container 2. An arc discharging voltage from an arc source 22 is applied between the filament 8 and the plasma production container 2. The arc discharge is generated between the filament 8 and the plasma production container 2. Thus, the organometallic gas 28 is ionized to generate the plasma 12. Then, the ion beam 16 can be led from this plasma 12. For example, when the organometallic gas 28 is used as the raw gas, the ion beam 16 containing indium ion or gallium ion can be led.
The reflecting electrode 10 repulses electron emitted from the filament 8 to serve as heightening ionization efficiency of the gas and generation efficiency of the plasma 12.
There are many cases that the organometallic gas 28 has strong reactivity by itself (trimethylindium is in this case) and that activated molecule or activated atom generated by changing the organometallic gas 28 into the plasma have strong reactivity. In the ion source where the organometallic gas 28 is introduced as it is into the plasma production container 2, there are problems that (1) parts such as the filament 8, reflecting electrode 10 and insulating materials 24, 25 in the plasma production container 2 are affected with quality alteration, whereby the amount of generating the plasma and the amount of generating the ion beam are altered so that lives of these parts are shortened, (2) dirt is easy to occur in the plasma production container 2, and by the dirt, insulating failures arise between the filament 8 and the plasma production container 2 and other parts, thereby resulting to disturb the stable actuation of the ion source, and (3) maintenance (disassembly, cleaning or the like) should be frequently done for removing the dirt.
To explain more specific examples, if the organometallic gas 28 is trimethylindium gas, there are following problems.
(1) The insulating capacity between the filament 8 and the plasma production container 2, more specifically of the insulating material 24 decreases by carbon occurring by decomposition of trimethylindium. Accordingly, the arc discharging voltage cannot be normally applied therebetween, and the amount of generating the plasma 12 and the amount of generating the ion beam 16 are altered to be unstable. The electron reflecting actuation at the reflecting electrode 10 is altered to be unstable also by decreasing of the insulating capacity of the insulating material 25 for the reflecting electrode 10. The amount of generating the plasma 12 and the amount of generating the ion beam 16 are made unstable.
(2) The filament 8 at high temperature is hydrogenated or carbonized and effected with quality alteration by activated hydrogen or activated carbon occurring through decomposition of trimethylindium. The amount of generating thermoelectron from the filament 8 is changed thereby, and the generating amount of the plasma 12 is changed and the generating amount of the ion beam 16 is changed correspondingly. The life of the filament 8 is also shortened.
(3) The filament 8 is embrittled by the activated hydrogen or the activated carbon occurring through brittleness decomposition of trimethylindium, and the amount of generating the thermo-electron from the filament 8 is changed. Thereby, the generating amount of the plasma 12 is changed and the generating amount of the ion beam 16 is also changed. The life of the filament 8 is shortened.
(4) For stabilizing and continuing the plasma 12 with only the trimethylindium gas being the raw gas, it is necessary to supply the trimethylindium gas more than required (that is, more than the amount required for obtaining a desired amount of the indium ion beam). Therefore, excessive indium or carbon existing in the plasma production container 2 increases, and dirt therein becomes larger. The interior of the plasma production container 2 should be frequently cleansed, otherwise the stable actuation of the ion source will be difficult.
(5) Since it is necessary to supply the trimethylindium gas more than required for stabilizing and continuing the plasma 12, the interior of the gas introducing pipe 26 is contaminated and easily clogged by indium metal caused by thermal decomposition of the gas before being supplied into the plasma production container 2. As a result, the stable supply of trimethylindium gas is difficult, and the production amount of the ion beam 16 becomes unstable.
Also in the case of the above-mentioned organometallic gases 28 other than the trimethylindium gas, similar problems arise as (1) to (2).
Furthermore, recently, attention has been paid to an indium ion implantation to substrates of a semi-conductor (for example, a silicone substrate or gallium arsenic substrate).
As an ion source to be used to, for example, such purposes, there is an ion source of so-called hot cathode type which uses the thermoelectron generated from the filament (hot cathode) so as to ionize a raw gas containing indium in the plasma production container for leading ion beam containing indium ion.
In a case that a gasified material of such as indium chloride (InCl3) is used as the raw gas to the ion source, there will arise problems as follows. Namely, since such compounds have deliquescence (property becoming liquid by absorption of moisture from the air), the inner wall of the plasma production container is instantly contaminated by melted substances. Accordingly, it is difficult to air-exhaust by vacuum the interior of the plasma production container and to produce the plasma. In addition, since acid is generated by melting, the inner wall of the plasma production container is corroded. Many troubles are taken for cleansing melted materials.
In a case that gasified materials of such as metallic indium (In) are used as the raw gas, since these materials are low in a steam pressure, there will occur a problem that an oven of high temperature for gasification (for example, heating temperature is around 800 to 1000xc2x0 C.).
On the other hand, trimethylindium [In(CH3)3] or triethylindium [In(C2H5)3] are high in the steam pressure to a certain extent. Therefore, it is not necessary to use the high temperature oven for gasification. As they have no deliquescence, the inner wall of the plasma production container is neither contaminated nor corroded. Because of such merits, it is very convenient to use these gases as the raw gas.
However, it was found that when the trimethylindium gas or the triethylindium gas was used as the raw gas in the ion source of the hot cathode type as above mentioned for leading the ion beam containing the indium ion, the filament was deteriorated in a short time (around 1 to several hours) and the serving live thereof ceased. For the filament, a wolfram filament ordinarily used in the ion source was used.
The deterioration process of the filament was examined as follows. As an example shown in FIG. 5, many voids (air holes) occur in the interior and surface of the filament 30, so that the surface is made rugged. When these voids occur and grow, a distribution in surface temperature of the filament 30 when driving the ion source gradually, becomes non-uniform, and at the same time, local deterioration of the filament 30 advances thereby, and one portion 34 is made thin. The non-uniformity in the temperature distribution further progresses, the portion 34 becomes rapidly thin, and consequently, the life of the filament 30 is acceleratedly shortened and goes to breaking of wire.
It was seen that when the trimethylindium gas or the triethylindium gas was used as the raw gas, much merit were available as mentioned above, but on the other hand, there was a serious problem that the life of the filament was short.
It is an object of the present invention to enable to stabilize actuation of the ion source, stabilize the amount of generating the ion beam, lengthen lives of composing parts and make maintenance easy.
It is another object of the present invention to enable to lengthen the life of the filament while making the best use of the merit of employing the trimethylindium gas or the triethylindium gas as the raw gas.
The ion source of the present invention comprising a gas introducing mechanism for introducing an inert gas and the organometallic gas into a plasma production container.
By the gas introducing mechanism, it is possible to introduce the inert gas and the organometallic gas being the raw gas into the plasma production container. As a result, the flowing amount of the organometallic gas can be lessened while securing the flowing amount of total gas necessary for stabilizing and continuing the plasma in the plasma production container and the amount of the ion beam by the sort of a desired ion.
Consequently, various problems arising in company with using of the organometallic gas can be reduced, and it is possible to enable to stabilize the actuation of the ion source, stabilize the amount of generating the ion beam, lengthen lives of composing elements and make maintenance easy.
Further, in the ion source of the present invention, a raw gas is trimethylindium gas or the triethylindium gas, and the filament comprises tantalum.
In the above mentioned gases other than the trimethylindium gas or the triethylindium gas, the rapid deterioration phenomenon of the wolfram filament was not seen. Therefore, this is considered as a phenomenon particular to the combination of the wolfram filament and the trimethylindium gas or the triethylindium gas.
Contemplating the reason therefor, it is assumed that activated hydrogen or activated carbon are generated by changing the trimethylindium gas or the triethylindium gas into plasmas, and they invade between metallic crystals of the wolfram filament heated at high temperature by their serving as the hot cathode, whereby many voids appear in the interior or the surface of the wolfram filament.
On the other hand, forming the filament with tantalum (Ta), it was confirmed that the live was very lengthened in comparison with wolfram (around 5 to 6 times as later mentioned).
Contemplating the reason therefore, it is assumed that the tantalum filament can occlude the activated hydrogen or the activated carbon as maintaining the state of metallic crystal. Therefore, voids are hard to occur in comparison with the wolfram filament. Tantalum can occlude hydrogen as 740 volume under e.g., a black-red heat, in other words, Tantalum can occlude 740 times as much hydrogen as its volume when is heated to glow black-red.